A single electrode has proven to elicit the perception of a spot of light, a so-called phosphene, in humans with vision impairment. U.S. Provisional Application 60/473,304, filed May 29, 2003 disclosed the implantation of electronic devices wholly or partially at the retina, with an array of electrodes provided to deliver electrical stimulation to remaining intact retinal neurons. An improved arrangement of electrodes was disclosed comprising a stimulation array whereby electrodes are arrayed, in whole or in part, in a staggered pattern allowing for a high density of phosphenes, but wherein the elicitation of discrete phosphene is nonetheless achievable. (See FIG. 1)
In such an array each electrode of said stimulation array is relatively large by comparison to remaining intact retinal neurons, stimulating many neurons when actuated. In such an array, an electrode primarily activates intact retinal neurons that lie on the small, retinal region directly adjacent to the center of said electrode. With increased stimulation, said region of activation increases, a phenomenon approximately modeled by circular regions of increasing radii, concentric to said adjacent region.
For an electronic retinal prosthesis, electrodes effectively render an image by way of phosphenes in the implant recipient's visual field. This is achieved by way of each electrode activating a population of retinal neurons in a discrete region; each population pertaining to the perception of a phosphene. A high density of rendered phosphenes (and therefore a high density of electrodes) is desirable for it allows better visual acuity in the implant recipient. This density, however, is constrained by interference. If any two regions of activation are too close, injected charge will interfere, meaning that the elicitation of discrete phosphenes can not be achieved. For example, an intraocular array of two, small, stimulating electrodes which, when actuated maximally, can activate two large, circular regions of retinal tissue. The two stimulating electrodes need be disparate enough such that the two, said circular regions do not interfere. However, since high density is desirable, the two electrodes should be close enough such that the two circular regions meet tangentially.
In light of the above, the problem as to how to array said stimulating electrodes is analogous to the geometric problem regarding optimally packing equi-sized circles on an unbounded plane. It is a geometric result that the densest packing of equi-sized circles on an unbounded plane is a mosaic exhibiting staggering between successive rows and columns, as illustrated by the four rows and seven columns in FIG. 1. An example of this mosaic is the hexagon.
A stimulating prosthesis of non-trivial complexity must be configured to deliver electrical stimuli. Typically this is achieved by way of switching, via a multiplexing circuit, current or voltage sources to the intended electrodes. Configuring said multiplexing circuit requires instructions to configure, time to convey, and time to act upon said instructions. It is therefore advantageous to reduce either or both the instructions necessary to configure the multiplexing circuit, or the time required for said instructions to be delivered and acted upon.
Utilizing the hexagonal mosaic for electrode layout, or abstracts thereof, a novel multiplexing method for configuring and delivering the stimulus or stimuli from said electrode layout is described.